1. Field of the Invention
The present invention relates to a fixing device that fixes a toner image onto a recording medium by an action of heat and pressure, and to an image forming apparatus including the fixing device.
2. Description of the Related Art
As a fixing device for use in an electrophotographic image forming apparatus such as a copying machine and a printer, a fixing device of heat-roller fixing type has been in wide use. The fixing device of heat-roller fixing type includes a pair of rollers (a fixing roller and a pressure roller) that are brought into contact with each other under pressure. By means of a heating section composed for example of a halogen lamp, which is placed in each of or one of the pair of rollers interiorly thereof, the pair of rollers are heated to a predetermined temperature (a fixing temperature). With the pair of rollers kept in a heated state, such as a recording paper sheet, which is a recording medium having formed thereon an unfixed toner image, is fed to a region where the pair of rollers make pressure-contact with each other (a fixing nip region). Upon the recording paper sheet passing through the pressure-contact region, the toner image is fixed to the recording paper sheet under application of heat and pressure.
Incidentally, a fixing device for use in a color image forming apparatus generally employs an elastic roller constructed by forming an elastic layer made for example of silicone rubber on a surface layer of the fixing roller. By designing the fixing roller as an elastic roller, it is possible for the surface of the fixing roller to become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the fixing roller makes contact with the toner image so as to cover the surface of the toner image. This makes it possible to perform satisfactory thermal fixing on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer in the fixing nip region, it is possible to provide enhanced releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner. Further, since the fixing nip region is convexly curved in a radially-outward direction so as to define a so-called reverse nip configuration, it is possible to attain higher paper-stripping capability. That is, a paper stripping action can be produced without using a stripping portion such as a stripping pawl (self-stripping action), wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
Incidentally, in such a fixing device provided in a color image forming apparatus, it is necessary to make a nip width of the fixing nip region wide in order to correspond to increase in speed. One available method of increasing the fixing nip width is to increase the thickness of the elastic layer of the fixing roller and the diameter of the fixing roller. However, in a fixing roller having an elastic layer, the elastic layer can not sufficiently conduct heat, thus, in a case where a heating section is provided inside the fixing roller, there is a problem that a temperature of the fixing roller is not followed when a process speed is increased. On the other hand, when a diameter of the fixing roller is increased, there is a problem that it takes longer time to warm up or power consumption is increased.
As a fixing device provided in a color image forming apparatus to solve such problems, Japanese Unexamined Patent Publication JP-A 10-307496 (1998) discloses a fixing device of belt fixing type that is configured so that a fixing belt is supported around a fixing roller and a heating roller, and the fixing roller and a pressure roller are brought into pressure-contact with each other with the fixing belt interposed therebetween. In the fixing device of belt fixing type, since the fixing belt with a small heat capacity is heated, it takes short time to warm up and it is not necessary to incorporate a heat source such as a halogen lamp in the fixing roller, thus making it possible to provide a thick elastic layer with low hardness made of sponge rubber and the like and to secure a wide nip width.
Furthermore, JP-A 2002-333788 discloses a fixing device of planar heat generating belt fixing type with a heating section as a planar heat generating element. In the fixing device of planar heat generating belt fixing type, when a heat capacity of the heating section is reduced, the planar heat generating element as the heating section directly generates heat at the same time, thus a thermal response speed is also enhanced compared to a system in which a heating roller is heated indirectly using a halogen lamp or the like and it is possible to attain further shortening of a time for warm up and more energy saving.
When the planar heat generating element composed of a resistance heat generating element including a metal and an inorganic substance is separated or risen from a substrate, there is a possibility that the planar heat generating element becomes an overheated state and results in smoke generation or burnout. Therefore, in the fixing device of planar heat generating belt fixing type, temperature abnormality under which the planar heat generating element becomes the overheated state is detected by an overheat preventing element (thermostat, thermal fuse, thermal protector, or the like), energization to the planar heat generating element is cut off based on the detection result, and thereby it is possible to prevent that the planar heat generating element becomes the overheated state and results in smoke generation or burnout.
In the fixing device of belt fixing type using the planar heat generating element of high power density, since the temperature rising speed of the planar heat generating element is high, in order to prevent that the planar heat generating element becomes the overheated state and results in smoke generation or burnout, the temperature abnormality under which the planar heat generating element becomes the overheated state should be detected earlier. In order to detect earlier the temperature abnormality under which the planar heat generating element becomes the overheated state, the overheat preventing element may be arranged to be in contact with the fixing belt or the planar heat generating element, however, in such a case, there is a possibility that not only a defect occurs in the fixed image on recording paper, but also temperature distribution on the surface of the fixing belt becomes non-uniform. Furthermore, when the overheat preventing element is arranged to be in contact with the fixing belt or the planar heat generating element, there is a possibility that detection sensitivity of the overheat preventing element becomes poor and thereby the thermal abnormality itself is not possible to be detected.
Moreover, in the fixing device of planar heat generating belt fixing type, a width of a paper non-passing region on the fixing belt surface varies depending on a size of the recording paper to be supplied to the fixing nip region. In the paper non-passing region that the recording paper does not contact on the surface of the fixing belt, since heat generated from the planar heat generating element will not be taken by the recording paper, a regional part of the planar heat generating element corresponding to the paper non-passing region becomes an excessive temperature rising state. In this way, when the planar heat generating element becomes the excessive temperature rising state regionally corresponding to the paper non-passing region, there is a case where the overheat preventing element that detects overheated state of the planar heat generating element operates erroneously.
To solve the above problem, Japanese Unexamined Patent Publication JP-A 2003-280413 discloses a heating device composed of a resistance heat generating element that generates heat due to being energized, including a paper passing portion corresponding to a paper passing region of recording paper and a paper non-passing portion which is a region part other than the paper passing portion and to which a thermo-protector (overheat preventing element) is arranged in a vicinity thereof, and the paper passing portion and the paper non-passing portion are electrically connected in series.
In a case where the heating apparatus disclosed in JP-A 2003-280413 is used as a heating section that heats the fixing belt in the fixing device of belt fixing type, overheated state of the paper passing portion is indirectly detected by temperature change in the paper non-passing portion of the resistance heat generating element due to energization, when the temperature in the paper non-passing portion reaches a predetermined temperature, it is possible that the thermo-protector cuts off the energization. That is, by using the heating apparatus disclosed in JP-A 2003-280413 as a heating section that heats the fixing belt, irrespective of the size of the recording paper to be supplied to the fixing nip region, the overheated state of the paper passing portion is able to be indirectly detected by the temperature change in the paper non-passing portion of the resistance heat generating element corresponding to the paper non-passing region that the recording paper does not contact all the time, it is possible to prevent that the overheat preventing element operates erroneously.
However, in the heating apparatus disclosed in JP-A 2003-280413, it is configured that the paper passing portion and the paper non-passing portion of the resistance heat generating element that generates heat due to being energized are electrically connected in series, when power of the paper passing portion and the paper non-passing portion is set to be the same, power density of the paper non-passing portion is smaller than that of the paper passing portion, and the temperature rinsing speed of the paper non-passing portion due to energization becomes slower than that of the paper passing portion. Therefore, even in the case where the temperature of the paper passing portion is risen to be the overheated state due to energization, the temperature of the paper non-passing portion shows a lower value than the paper passing portion, and thereby the overheat preventing element arranged in the vicinity of the paper non-passing portion is incapable of indirectly detecting the overheated state of the paper passing portion accurately from the temperature change in the paper non-passing portion due to energization. Furthermore, in the configuration where the paper passing portion and the paper non-passing portion are electrically connected in series, when the power density of the paper passing portion and the paper non-passing portion are set to be the same, the power of the paper non-passing portion is smaller than that of the paper passing portion, and thereby the overheat preventing element arranged in the vicinity of the paper non-passing portion is incapable of indirectly detecting the overheated state of the paper passing portion accurately from the temperature change in the paper non-passing portion due to energization.
Moreover, the configuration in which the paper passing portion and the paper non-passing portion are electrically connected in series is susceptible to a disturbance factor such as variation in applied voltage to the respective portions, and thus the temperature change in the respective portions is not the same, thus it is impossible to indirectly detect the overheated state of the paper passing portion accurately from the temperature change in the paper non-passing portion due to energization.